I dont think Slackware comes with an SMP kernel.
But its easy to do. Slackware will install just fine w/o it (though it will only use one processor).
Once its installed, you can then either download a kernel w/ SMP enabled from kernel.org , or enable it yourself in the current kernel.
Then rebuild the kernel, and your done.
You do need the Slackware kernel source if you want to use the current kernel.
#Kernel Source for Slackware 9:
install using "#installpkg kernel-source-2.4.20-noarch-5.tgz"
For a tutorial on compiling the kernel, check out the great guide in the top of the Slackware forum here.